Interactive detection of laser light plane (LPP) system has been known as an interactive detection technology for a projector and a large screen (refer to PTLs 1 and 2). As the interactive detection technology, a method is known in which an irradiation optical system generates a thin light film on a detectable surface such as a projection surface of the projector. When an object such as a finger passes through the thin light film, scattering light occurs at a site of the light film through which the object passes. The scattering light is detected as detected light by a camera. This makes it possible to determine a position of the object to perform, for example, an interactive operation. In the interactive detection technology, an infrared ray is commonly used to make the irradiation light invisible.